fix: lost references to the files.add method changed to files.addAll

[zebateira]

The only affected tutorial is "Regular Files API". Affected lessons are lessons number 3, 5 and 6.

[zebateira]

From the convo with @achingbrain

the plan for the future is to make it throw if you pass input that would result in multiple files being created

So I propose leaving the issue as it is: a user will be able to use ipfs.add(files). In the future we will get a release that will make that specific case fail, and then we'll be able to show an appropriate error message to the user.

[terichadbourne]

Merging as is for now, noting that we'll have to come back and update validation when the js-ipfs team settles on some more under-the-hood edits to how these methods work. See #498.